High voltage pulse transmission system employing field emission diodes for successively pulsing a load



2 Sheets-Sheet 1 W Q E32 v mm vw mm w r :33 m2; m?

P. HARBOUR mmamooum Jan. 9, 1968 J.

HIGH VOLTAGE PULSE TRANSMISSION SYSTEM EMPLOYING FIELD EMISSION DIODESFOR SUCCESSIVELY PULSING A LOAD Filed Jan. 25, 1965 JOHNR HARBOURINVENTOR BY BUCKHORN, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS 9, 1968 JP. BARBOUR 3,363,102

HIGH VOLTAGE PULSE TRANSMISSION SYSTEM EMPLOYING FIELD EMISSION DIODESFOR SUCCESSIVELY PULSING A LOAD Filed Jan. 25, 1965 2 Sheets-Sheet 2 MBR mm N n M LO/II J N9 Y 5 m a I 1| g d 1 g I \\\\L BUCKHORN, BLORE,KLAROU/ST a SPAR/(MAN ATTORNEYS Patented Jan. 9, 1968 3,363,102 HIGHVOLTAGE PULSE TRANSMISSION SYSTEM EIVIPLOYING FIELD EMISSION DIODES FORUCCESSIVELY PULSING A LOAD John P. Barbour, McMinnville, Oreg., assignorto Field Emission Corporation, McMinnville, Oreg., a corporation ofOregon Filed Jan. 25, 1965, Set. N0. 427,653 8 Claims. (Cl. 250-102)ABSTRACT OF THE DISCLOSURE An electrical pulse transmission system ofhigh frequency and power capability is described in which a plurality ofsequentially triggered high voltage pulse generators are connected to acommon load, such as a field emission X-ray tube, through a plurality offield emission diode switches which isolate the pulse generators fromeach other. The diode switches each have a cathode in the form of aplurality of sharp electron emitting ele ments and are capable ofcombined field emission and vacuum arc operation to transmit pulses ofhigh current.

The subject matter of the present invention relates generally to thetransmission of electrical pulses of high voltage and high current froma plurality of sources to a common load as a train of successive pulses,and specifically to a high voltage pulse transmission system including aplurality of field emission diode switches connected between the commonload and the outputs of a plurality of energy discharge pulse generatorsin order to electrically isolate such pulse generators from each otherbecause they are triggered at different times. Each diode switch has aplurality of field emission cathode elements to transmit a high voltage,high current pulse through such switch in a forward direction by acombination of field emission and vacuum arc operation and to block thetransmission of pulses through the switch in the reverse direction.

The pulse transmission system of the present invention -is especiallyuseful when employed to transmit electrical pulses to an X-ray tube forproducing narrow, short rise time pulses of X-rays of high intensity.Such an X-ray system may employ a plurality of electrical pulsegenerators similar to that disclosed in co-pending US. patentapplication, Ser. No. 245,182, entitled X-ray Unit, filed by W. P. Dykeet al., Dec. 17, 1962, now US. Patent 3,256,439, whose outputs areconnected through different field emission rectifier switches to acommon load in the form of an X-ray tube of the field emission cathodetype shown in the above-mentioned co-pending application. However, thepulse transmission system of the present invention may be used withother common loads requiring high energy electrical pulses of highvoltage and high current such as various microwave devices, includingmagnetrons and klystrons.

The pulse transmission system of the present invention has severaladvantages over previous systems, including a faster repetition ratewhich is not limited by the charging time of the electrical pulsegenerators. As a result the present system can produce a train ofelectrical pulses having a frequency up to about 1 megacycle per second,which is the switching time of the field emission diode switches. Inaddition, the pulse transmission system herein described transmitspulses of higher current and voltage having lower rise times due to theuse of such switches. For example, one commercial embodiment of thepresent system transmits rectangular output pulses of 150 kilovolts and2000 amperes amplitude having a pulse width of about 70 nanoseconds anda rise time of about 14 nanoseconds with the spacing between pulsesbeing about 30 to 50 microseconds.

When these electrical pulses are applied to a load in the form of anX-ray tube of the field emission, vacuum arc type, such tube emits atrain of narrow X-ray pulses of high intensity and short rise timehaving high dosage rates. These X-ray pulses can be employed to recordthe images of rapidly moving objects or fast happening events directlyon large sections of film or other recording device without electronicintensification to provide images of better resolution. Such an X-raysystem is also simpler and less expensive than previous systemsemploying a continuous X-ray source of low intensity, and an imageintensifier tube. In addition, no camera shutter mechanism is needed inthe present system because of the high frequency of the pulsing X-raysource. This also allows the film to be moved continuously duringexposure such as by rotating a strip of film on a circular drum totransport the film sections into position. In addition, such an X-raysystem employs only a single X-ray tube, rather than a plurality ofpulsed X-ray sources as employed in some previous systems, therebyeliminating any image distortion due to different exposure angles ofsuch previous systems.

It is therefore one object of the present invention to provide animproved system for transmitting electrical pulses of high voltage andhigh current to a common load from a plurality of different sources inthe form of a train of electrical pulses of a high repetition rate.

Another object of the present invention is to provide an improved pulsetransmission system employing field emission diode switches between acommon load and a plurality of separate triggered electrical pulsegenerators to isolate such pulse generators from each other.

A further object of the present invention is to provide an improvedpulse transmission system of simple and inexpensive construction fortransmitting a plurality of narrow electrical pulses of high voltage andhigh current successively from a plurality of different pulse generatorsof the energy storage type to a common load in the form of a train ofpulses having a high repetition rate which is not limited by thecharging time of such pulse generators.

An additional object of the present invention is to provide an improvedsystem for producing a series of narrow X-ray pulses of high intensityand high dosage rate at a fast repetition rate from a single source ofXrays.

Still another object of the present invention is to provide a system forproducing a series of X-ray pulses of high intensity by transmitting atrain of electrical pulses of high voltage and high current to an X-raytube of the field emission, vacuum arc type from a plurality oftriggered pulse generators through diode switches of the field emission,vacuum arc type.

A still further object of the invention is to provide an improvedradiographic system producing a plurality of X-ray pulses of greatintensity and high dosage rates to record the images of rapidly movingobjects or fast happening events in a simple and inexpensive manner toproduce high resolution images Without the use of electronic imageintensification or camera shutters.

Additional objects and advantages of the present invention will beapparent from the following detailed descriptions of preferredembodiments thereof and from the attached drawings, of which:

FIG. 1 is a schematic diagram of an X-ray apparatus including anelectrical pulse transmission system in accordance with the presentinvention;

FIG. 2 is a longitudinal section view of one embodiment of a fieldemission diode switch employed in the system of FIG. 1;

FIG. 3 is a longitudinal section view of another embodiment of a diodeswitch employed in the system of FIG. 1, shown on-an enlarged scale; and

FIG. 4 is a magnified partial vertical section View taken along the line44 of FIG. 3.

One embodiment of the electrical pulse transmission system of thepresent invention is shown in FIG. 1 to include a plurality of triggeredelectrical pulse generators 10, 12 and 14, of the energy storage anddischarge type. The outputs of each of the pulse generators 10, 12 and14 are connected through coaxial cabletransmission lines 16, 18 and 19,respectively, to the input terminals of a plurality of field emissiondiode switches 20, 22 and 24, respectively. Each of these diode switcheshas a field emission cathode structure including a plurality of pointedneedles orother cathode elements having emitter portions of small radiusof curvature to enable the field emission of electrons from suchelements. The structure of such diode switches is hereafter described ingreater detail with reference to FIG. 2 or to FIGS. 3 and 4. The diodeswitches are mounted within X-ray shielded tube heads 28, 3G and 32,respectively, which each plug into a common junction box 34 so that theoutput terminals of the diode switches are connected together to acommon output terminal on such junction box. The output terminal of thejunction box 34 is connected througha coaxial cable transmission line 36to an X-ray tube 38.

The X-ray tube 38 may have a field emission cathode structure includinga plurality of sharp pointed needle elements 49 of tungsten supported inradially spaced relationship about a conical anode 42 of tungsten in themanner of co-pending U.S. patent application Ser. No. 245,182, now US.Patent 3,256,439. When an electrical pulse is transmitted to the X-raytube from one of the pulse generators 16, 12 and 14 through one of thediode switches 20, 22 and 24, electrons are emitted from at least someof the cathode needles 40 due to the high electrical field surroundingthe points of such needles and some of the needles are partiallyvaporized to produce a vacuum are between the anode and cathode. Theelectrons are accelerated and focused to bombard the conical anode 42 toemit X-rays 44 from such anode which are transmitted through a thinwindow in the end of the evacuated tube envelope, adjacent the apex ofthe conical anode. Short X-ray pulses of extremely high intensity areproduced because "of the vacuum arc operation of the X-ray tube whichgreatly increases the current transmitted-between the cathode structureand the anode to a value of about 1,000 to 2,000 amperes. This extremelyhigh current flows only for the short time that an electrical pulse isapplied .to the X-ray tube and is terminated before any ions ofvaporized anode metal are transmitted to the cathode elements to preventsuch ions from distorting the needle shape of such elements. The highcurrent of vacuum are operation is caused by positive ions of cathodemetal neutralizing the negative space charge normally surrounding suchneedles to enable a much greater current to flow to the anode.

The X-ray tube 38 may be positioned adjacent the path of a moving object46, such as a bullet or exploding wire, so that such object ispositioned between the X-ray tube and a recording device 48 to exposethe object to the X-rays and produce a separate image of the object onthe recording devicefor each X-ray pulse. Any number of a plurality ofdifferent types of recording devices can be employed in the radiographicsystem of the present invention. For example, the recording device 48can be merely a single fixed X-ray film to record the path of the object46 by multiple exposure of such film to a plurality of consecutive X-raypulses. Alternatively, the recording device can be in the form of aplurality of moving film sections mounted on a rotating drum or othercarrier so that a different one of such film sections is exposed foreach X-ray pulse. A faster recording device can be formed by employingan X-ray sensitive, fluorescent screen and a plurality of separate lightsensitive films focused on such screen through different light paths,each including a voltage actuated light valve, such as a Kerr cell,which is opened at a time corresponding to the triggering of one of thepulse generators for a brief time sufiicient to expose the filmassociated with such valve to the X-ray pulse produced thereby. Inaddition, other conventional recording devices can be employed including56, respectively. The outputs of the storage modules are connected inseries through a plurality of spark gaps which are ionized by causingultra-violet light to be emitted from the ionized gas in one spark gapto all of the remaining spark gaps substantially instantaneously whensuch one spark gap is caused to break down by the application of anelectrical trigger pulse in a manner hereafter described. All of thepulse generators 10, 12 and 14 are connected to a common source 58 ofpressurized nitrogen gas which is employed in the spark gaps because itemits a large quantity of ultra-violet light when ionized. It isbelieved that the ultra-violet light emitted from; the triggered sparkgap causes photo-electrons to be emitted from the electrodes of theremaining spark gaps which together with the ionizing effect of theultra-violet light produces the breakdown of the remaining gaps.

The trigger input terminal of each of the pulse generat-ors 10, 12 and14 is connected to the secondary winding of voltage step up transformers60, 62 and 64, respectively, whose primary winding is connected to theoutput of trigger amplifier and variable time delay circuits 66, 68 and70, respectively. The input terminals of the amplifiers 66, 68 and 70are connected to a common trigger input terminal 72 to which a triggerpulse is applied at a time corresponding to the movement of the object46 past the X-ray tube. As a result of the difierent time delays ofcircuits 66, 6-8 and 70', the trigger pulse is transmitted to the pulsegenerators 10, 12 and 14 at dif- D ferent times corresponding todifferent positions of the object as it passes between the X-ray tubeand the recording device. The transformers '60, 62 and 64 increase thevoltage of such trigger pulses to about 10 kilovolts before applyingthem to such pulse generators. Each of the circuits 66, 68 and 70 has avariable resistor 74, 76 and 78, respectively, for varying the timedelay of the trigger pulse transmitted through such circuit.

One embodiment of the diode switch employed for switches 26, 22 and 24is shown in FIG. 2 to have a planar electrode structure and includes ananode S0 in the form of a fiat circular disc of metal attached tosupport pins 81 and 82 extending through a seal in the reentrant end ofan evacuated glass envelope 84. A field emission cathode structure 86,including a plurality of spaced needle elements 88 of tungsten extendingfrom the side of a common circular support disc 90 toward the anode, isattached at the opposite side of such support disc to a support rod 92.The support rod extends through the other end of the envelope and issealed to such envelope by an annular metal flange 94 attached to suchrod. A filament wire 96 coated with getter material may be providedwithin the tube envelope adjacent the rear surface of the anode 80 toflash the getter material onto the envelope wall only adjacent thereentrant end; One end of the getter wire 96 is connected to anodesupport pin 82 and its other end is connected to another support pin 97to enable the transmission of heating current through such getter wirethrough leads 9S and 100 respectively connected to support pins 82 and97. After the gettering operation, lead 100 is disconnected, and lead 98is connected as the input signal conductor of the diode switch whenpositive voltage pulses are applied thereto. Of course, the cathodesupport rod 2 would serve as the input conductor of the switch ifnegative voltage pulses were applied thereto.

The diode switch shown in FIG. 2 has a vacuum arc operation similar tothat described above with regard to the X-ray tube 38 except that thematerial of the anode is a metal having a high atomic number to preventthe emission of X-rays. Any X-rays which are emitted from the rectifiertube are absorbed by tubular lead shielding 102 surrounding such tubewhich is provided within the tube heads 28, 30' and 32 of FIG. 1. When apositive voltage pulse is applied to anode 80 or a negative Voltagepulse is applied to the cathode structure 85, a strong electrical fieldis produced adjacent the points of the cathode needles 88 to cause thefield emission of electrons from the points of at least some of suchneedles. It should be noted that each of the needle points has a radiusof curvature on the order of centimeters or less and is spaced about3.75 millimeters from the anode. Portions of at least some of suchneedles are melted to produce positive ions of metal vapor between thecathode needles and the anode. These positive ions neutralize thenegative space charge usually surrounding the cathode needles and enablea much greater flow of current of the order of 1000 to 2000 amperesbetween the cathode structure and anode of the switch tube. Since thepoints of certain of the cathode needles will be positioned slightlycloser to the anode than others or will have smaller radii of curvature,these needles will emit electrons for a given applied electrical pulsewhile the other needles will not. In this manner, the lifetime of thetube is extended to several thousand pulses.

When the positive pulses of 150 kilovolts are applied to the cathode ofthe diode switches, no current flows in the reverse direction from theoutput to the input terminals of such switches because of the lowerintensity electrical field produced adjacent the anode 80 which is below5 l0+ volts per centimeter. This is substantially less than the highelectrical field produced adjacent the cathode needles which is greaterthan S X10+ volts per centimeter when such positive pulse is applied tothe anode of the switches. Thus, a high voltage pulse produced by pulsegenerator 10 and transmitted through switch tube 20 in the forwarddirection is not passed in the reverse direction through switch tubes 22and 24 but is transmitted to the X-ray tube 38 instead. In this manner,the diode switches isolate the pulse generators from each other.

Another embodiment of the rectifier switch is shown in FIGS. 3 and 4 andis similar to the switch of FIG. 2 but has a coaxial electrodestructure. The anode 80' is in the form of a circular cylinder, and thecathode structure 86 includes a plurality of separate support blocks 90'each having needle elements 88' extending therefrom. The cathode blocks90 are uniformly spaced about the anode 80' with the cathode needlesdirected radially inward toward such anode. Each of the cathode blocks90' is attached by welding within a longitudinal fold 104 in a commonsupport tube 106 of metal which is secured in a similar manner to acircular end plate 108 of metal forming a portion of the evacuated tubeenvelope. The end plate 108 is welded to one end of a metal tube 110which is secured at its other end to the glass envelope 84' by aconventional glass-to-metal seal. A tubular shield member 112 isattached to support 106 around the anode in such a position to preventany metal vapor emitted from the cathode needles 88' or such anode fromstriking the glass envelope 84 in the region immediately adjacent theglass-to-metal seal between such envelope portion and the metal endportion 110 to prevent short circuiting the anode to the cathode. Theconstruction of the cathode structure including the support blocks 90'and the needle elements 88 may be similar to that disclosed inco-pending U.S. patent application, Ser. No. 114,125, filed June 1, 1961by W. P. Dyke et al., now U.S. Patent 3,174,043. It should be noted thatthe metal vapor deposited on the inner surface of the glass envelopeportion 84 due to melting of the cathode needles or anode acts as agettering material to maintain the vacuum within such tube at a highlevel.

The coaxial construction of the switch tube of FIGS. 3 and 4 enables theforward conduction impedance of such tube to more nearly match thecharacteristic impedance of the coaxial cables to which it is connectedto minimize signal reflections. Thus, in one embodiment the outputimpedance of each of the pulsers 10, 12 and 14 and the characteristicimpedance of each of the coaxial cables 16, 18 and 19 is ohms as well asthe forward conduction impedance of the switches 20, 22 and 24. Inaddition the reverse impedance of the switch is about 1,000 times thatof the forward impedance of such switch or about 75,000 ohms in order toprovide good isolation between the pulse generators.

It is obvious that many changes may be made in the details of theabove-described preferred embodiments of the present invention withoutdeparting from the spirit of the invention. For example, otherconfigurations of rectifier switches can be employed instead of thoseshown in FIGS. 2 to 4, such as a tube having a hollow cylindrical anodesurrounding a coaxially mounted cathode structure or a tube employing asingle flat disc having a razor edge type emitting portion as itscathode structure surrounding or being surrounded by the anode.Therefore the scope of the present invention should only be determinedby the following claims.

I claim:

1. A system for transmitting pulses of high voltage and currentsuccessively from a plurality of different sources a common load,comprisirw:

a plurality of electrical pulse generators each producing a pulse ofhigh voltage and high current when said pulse generators are triggeredand having a recovery time before the pulse generator is capable ofproducing another pulse;

trigger means for selectively triggering said pulse generators to causethe production of said pulses at different times so that said pulses arespaced apart and the time between successive pulses can be less than therecovery time of one pulse generator;

a common load; and

a plurality of rectifier switches having their output terminalsconnected in common to said load and their input terminals connected todiiferent ones of said pulse generators to transmit said pulses to saidload and to isolate said pulse generators from each other, each of saidswitches including an anode and a field emission cathode structure.

2. A system for transmitting pulses of high voltage and currentsuccessively from a plurality of diiferent sources to a common load,comprising:

a plurality of electrical pulse generators each producing a pulse ofhigh voltage and high current when said pulse generators are triggeredto discharge the electrical energy stored in said pulse generators;charge means for charging said pulse generators to a predetermined DC.voltage in a given charging time;

trigger means for selectively triggering said pulse generators to causethe production of said pulses at different times so that said pulses arespaced apart and the time between successive pulses can be less than thecharging time of one pulse generator;

a common load; and

a plurality of rectifier switches having their output terminalsconnected in common to said load and their input terminals connected todiiferent ones of said pulse generators, each of said switches includingan anode and a field emission cathode structure includa 7 ing aplurality of electron emitting elementssof small radius of curvaturedirected toward said anode.

3. A system for transmitting pulses of high voltage and currentsuccessively from a plurality of different sources to a common load,comprising: V

a plurality of electrical pulse generators each producing a pulse ofhigh voltage and high current when said pulse generators are triggeredand having a recovery time before the pulse generator is capable ofproducing another pulse; v trigger-means for selectively triggering saidpulse generators to cause the production of saidpulses at differenttimes so that said pulses are spaced apart and the time betweensuccessive pulses can be less than the recovery time of one pulsegenerator;

a common load; and

a plurality ofrectifier switch tubes having their output terminalsconnected in common to said load and their input terminals connected todifferent ones of said pulse generators, each of said switch tubesincluding an anode and a field emission cathode a structure mountedwithin an evacuated envelope, said cathode structure including aplurality of electron emitting elements of small radius of curvaturesupported in proper spaced relationship'withurespect to said anode andsaid pulses being of sufiicient amplitude to cause the field emission ofelectrons from at least some of the cathode elements to said anode andto produce a vacuum are between said anode and'said cathode elementswhen one of said pulses is applied to its associated switch tube by thepulse generator connected to its input terminal so that said one pulseis transmitted through said associated switch tube to said load but toprevent the pulses produced by the other pulse generators from beingtransmitted through said associated switch tube away from said load.

4. A system for transmitting pulses of high voltage and currentsuccessively from a plurality of different sources to a common load,comprising:

a plurality of electrical pulse generators each producing a pulse ofsubstantially the same high voltage and high current when said pulsegenerators are triggered to discharge the electrical energy stored insaid pulse generators;

charge means for charging said pulse generators to the samepredetermined DC. voltage in a given charging time; a

trigger means for selectively triggering said pulse generators to causethe production of said pulses at different times so that said pulses arespaced apart and the time between successive pulses can be less than thecharging time of one pulse generator;

a common load; and

a plurality of rectifier switch tubes having their output terminalsconnected in common to said load and having their input terminalsconnected to diiferent ones of said pulse generators, each of saidswitch tubes including an anode and a field emission cathode structuremounted within an evacuated envelope, said cathode structure including aplurality of needle elements with points of small radius of curvaturesupported in proper spaced relationship with respect to said anode andsaid pulses being of sufficient amplitude to cause the field emission ofelectrons from at least some of the needle elements to the anode and toproduce a vacuum are between said anode and said needle elements whenone of said pulses is applied to its associated switch tube by the pulsegenerator connected to its input terminal so that said one pulse istransmitted in a forward direction through said associated switch tubeto said load but to prevent the pulses produced by the other pulsegenerators from being transmitted in a reverse directo produce aplurality of short X-ray pulses of high intensity and fast repetitionrate, comprising:

a plurality of electrical pulse generators each producing a pulse ofhigh voltage and high current when said pulse generators are triggeredto discharge the electrical energy stored in said pulse generators;

charge means for charging said pulse generators to a predetermined DCvoltage in a given charging time;

trigger means for selectively triggering said pulse generatorssuccessively to cause the production of said pulses at difierent timesso that the time between V successive pulses is less than the chargingtime of one ulse generator;

an X-ray tube having an anode and a field emission cathode structure;and r a plurality of rectifier switch tubes having their outputterminals connected in common to said X-ray tube and their inputterminals connected to different ones of said pulse generators, each ofsaid switch tubes including an anode and a field emission cathodestructure, said cathode structure having a plurality of spaced emittingelements of small radius of curvature directed toward said anode.

6, An X-ray system employing a single X-ray source to produce aplurality of short X-ray pulses of high intensity and fast repetitionrate, comprising:

a plurality of electrical pulse generators each producing a pulse ofhigh voltage and high current when said pulse generators are triggeredto discharge the electrical energy stored in said pulse generators;

charge means for charging said pulse generators to a predetermined DC.voltage in a given charging time;

trigger means for selectively triggering said pulse generatorssuccessively to cause the production of said pulses at diiferent timesso that the time between successive pulses can be less than the chargingtime of one pulse generator;

an X-ray tube having an anode and a field emission cathode structureincluding a plurality of spaced electron emitting elements;

a plurality of rectifier switch tubes having their output terminalsconnected in common to said X-ray tube and their input terminalsconnected to different ones of said pulse generators, each of saidswitch tubes including an anode and a field emission cathode structuremounted within an evacuated envelope, said cathode structure having aplurality of spaced emitting elements of small radius ofcurvature; and

means for applying said electrical pulses to the input terminals of eachof said switch tubes to cause the field emission of electrons from atleast some of said emitting elements to said anode in such switch tubeand to vaporize portions of said elements and produce ions of cathodematerial which form a vacuum are between the anode and cathode structurethat greatly increases the current of the electrical pulse transmittedin a forward conduction direction through said switch tube, and fortransmitting said electrical pulses from the output terminals of saidswitch tubes to said X-ray tube to produce an X-ray pulse for eachelectrical pulse, while preventing said electrical pulses from beingtransmitted in the reverse conduction direction through other switchtubes.

7. A Cineradiographic system employing a single X-ray source to producea plurality of short X-ray pulses of high intensity and fast repetitionrate, comprising:

trigger means for selectively triggering said pulse generatorssuccessively to cause the production of said pulses at different timesso that the time between successive pulses can be less than the chargingtime of one pulse generator;

an X-ray tube having an anode and a field emission cathode structureincluding a plurality of spaced electron elements, which emits a pulseof X-rays when one of said voltage pulses is applied to said tube;

a plurality of rectifier switch tubes having their output terminalsconnected in common to said X-ray tube and their input terminalsconnected to different ones of said pulse generators and each of saidswitch tubes including an anode and a field emission cathode structurewithin an evacuated envelope, said cathode structure having a pluralityof emitting elements directed toward said anode; and

means mounted adjacent said X-ray tube for receiving said X-ray pulsesafter they have passed through a moving object in order to record themovement of said object.

8. An X-ray system employing a single X-ray source to produce aplurality of short X-ray pulses of high intensity and fast repetitionrate, comprising:

.a plurality of electrical pulse generators each producing a pulse ofhigh voltage and high current when said pulse generators are triggeredto discharge the electrical energy stored in said pulse generators;

charge means for charging said pulse generators to a predetermined DC.voltage in a given charging time;

trigger means for selectively triggering said pulse generatorssuccessively to cause the production of said pulses at different timesso that the time between successive pulses can be less than the chargingtime of one pulse generator;

an X-ray tube having an anode and a field emission cathode structureincluding a plurality of spaced electron emitting needles;

a plurality of rectifier switch tubes having their output terminalsconnected in common to said X-ray tube and their input terminalsconnected to ditferent ones of said pulse generators, each of saidswitch tubes including an anode of a circular cylinder shape and a fieldemission cathode structure mounted coaxially about said anode within anevacuated envelope, said cathode structure having a plurality ofemitting elements of needle shape directed inward toward said anode; and

means for applying said electrical pulses to the input terminals of eachof said switch tubes to cause the field emission of electrons from atleast some of said emitting elements to said anode in such switch tubeand to vaporize portions of said elements to produce a vacuum arebetween the anode and the cathode structure which greatly increases thecurrent of the electrical pulse transmitted in a forward conductiondirection through said switch tube, and for transmitting said electricalpulses from the output terminals of said switch tubes to said X-ray tubeto produce an X-ray pulse for each electrical pulse, while preventingsaid electrical pulses from being transmitted in the reverse conductiondirection through other switch tubes.

References Cited UNITED STATES PATENTS 2,923,818 2/1960 Wilson et al.32861 2,942,190 6/ 1960 Fischman 32861 3,051,906 8/1962 Haynes 328613,171,030 2/1965 Foster et al. 250-- WILLIAM F. LINDQUIST, PrimaryExaminer.

RALPH G. NILSON, Examiner.

A. L. BIRCH, Assistant Examiner.

